Pneumolysin, a thiol-activated cytolytic toxin produced by many disease isolates of S. pneumoniae, is considered to be involved in the pathogenicity of pneumococcal infection by inhibiting the defense mechanism of the host resistance. We have prepared the purified antiserum against type 1 pneumolysin, isolated from an E. coli clone containing PJCP20 plasmid, and used this antiserum to examine the distribution of pneumolysin in various pneumococcal strains and cross- reactive bacteria. The amino acid sequence of type 1 pneumolysin was analyzed. The sequence of the first 39 amino acids of this protein was found to be the same as that reported in the literature. Expression and distribution of pneumolysin were examined in various pneumococcal disease isolate types, non-pathogenic types and cross- reactive bacteria. The pneumolysin was present in pneumococcal types 1, 2, 3, 5, 6B, 7, 9A, 9L, 9N, 9V, 10, 11, 12F, 15B, 17F, 19A, 19B, 19C, 19F, 20, 22, and 23F and non-capsulated strains #3095, 3096, and 3097. It was detected in the type 8 culture medium, but not in the cells. The specific hemolytic activity of encapsulated pneumococci ranged between 216 HU/mg protein in type 15B and 776 HU/mg protein in type 19F. The non-capsulated strains showed 293-502 HU/mg of activity. The function of pneumolysin in the mechanism of pathogenicity of virulent pneumococci and its role in the non-capsulated organisms will be examined further.